1. Field of the Invention
The invention relates generally to the field of amplifying devices for radio-frequency signals, and more specifically to amplifying systems for eliminating or overcoming dead spots in cellular radio-telephone systems caused by obstructions such as buildings or hills that block cellular radio-telephone signals in at least some portions of a cell.
2. Description of the Prior Art
Cellular radio-telephone systems have recently been introduced in a number of areas to provide telephone coverage for people who need to have access to telephones from outside of their offices or are otherwise unable to get to telephones that are hard-wired to a central office over conventional telephone lines.
In cellular radio-telephone systems, an area is divided into a plurality of small regions, or "cells", each of which is covered by a relatively low-power transmitter. Currently, cellular radio telephone service is provided in the 825 to 845 MHz and 870 to 890 MHz frequency bands. The higher freqency band is used for "down-link" transmissions from the "cell site" for reception by the subscriber. The "cell site" is the location of the transmitter, or, more specifically, the location of the antenna from which transmissions are effected for the cell. The lower frequency band is used for "up-link" transmissions from the subscriber in the cell for reception by the receiving equipment which is also located at the cell site.
Each frequency band assigned to the cellular radio telephone system is divided into two parts, with one part being reserved for the local telephone company and the other half being franchised to a competing service provider. Each channel has a thiry kilohertz bandwidth, allowing for 666 channels in each twenty megahertz band, with 333 being provided to the telephone company and the same number to the franchisee.
Since a cellular radio telephone system uses relatively low power and since the wavelengths of the signals are short, obstructions such as buildings and mountains which may be present between the cell site and a subscriber at various locations in a cell, can cause significant degradation in the signal levels, in some areas reducing them to unusable levels. Increasing the power of the signals may raise them to levels which are acceptable in those areas, but that could cause several problems. First, while adjacent cells do not use the same channels, at least some of the next closest cells will use the same channels, and raising power in some cells will cause interference in those other cells. Furthermore, raising the power of a signal in one channel may cause adjacent channel interference between channels in adjacent cells.
In any event, increasing power of the signal transmitted from a cell site will do nothing to enhance the signal the cell site receives from the subscriber. Indeed, the amount of power that a subscriber can transmit is limited at least by the capacity of battery, specifically in case of a portable telephone subscriber.
U.S. patent application Ser. No. 787,332 discloses several embodiments of a cell enhancer system having amplifiers in various configurations for receiving, amplifying and re-transmitting down-link signals from a cell site into an obstructed area, and also for receiving up-link signals from subscribers in the obstructed area, amplifying them and re-transmitting the amplified signals to the cell site. In one embodiment disclosed in that application, a single wide-band amplifier has an input terminal that receives both up- and down-link signals from antennas through a duplexer network, and that transfers through another duplexer network amplified up- and down-link signals to appropriate antennas for transmission. The wide-band amplifier amplifies all signals in at least the two cellular radio telephone bands.
Problems may arise with the above-described cell enhancer, since the enhancer amplifies signals in the ranges allocated to both the local telephone company and the franchisee. The actual area shielded by an obstruction depends upon the location of the cell site, and, since the telephone company and the franchisee may select different cell sites within a cell, the pattern of obstructed areas within a cell may differ as between the telephone company and franchisee. For example, an area that may be obstructed for the cell site of the franchisee may not be obstructed for the cell site of the telephone company. However, a cell enhancer as described above will transmit into the obstructed area down-link signals in the range for both the franchisee and the telephone company, which may interfere with direct signals from the telephone company's cell site.